In the related art, piezoelectric speakers using a piezoelectric vibrator in which a piezoelectric element is attached to a metal plate are known. Since piezoelectric speakers are thin and simple in structure as compared to dynamic speakers, piezoelectric speakers have advantages in that they can be miniaturized and are less expensive. However, piezoelectric speakers have disadvantages in that although they have a high sound pressure level near the resonance frequency thereof, the sound pressure level at other frequencies, particularly in a low-frequency domain, is low. In this specification, a low-frequency domain (hereinafter referred to as a low-frequency band) indicates frequencies of about 1 kHz or less, and a high-frequency domain (hereinafter referred to as a high-frequency band) indicates frequencies over about 1 kHz. However, there is no definite boundary between the low-frequency band and the high-frequency band.
Moreover, a piezoelectric speaker in which a piezoelectric vibrator is held by a film-shaped body formed of a resin to thereby increase a sound pressure level at a low-frequency band is known (for example, see Patent Document 1). Moreover, a piezoelectric audio device in which a metal plate for adjusting a resonance frequency is attached to a piezoelectric vibrator to thereby increase a sound pressure level at any frequency is known (for example, see Patent Document 2).
However, in such a piezoelectric speaker, the sound pressure level at the low-frequency band is still low, and it is not possible to obtain a sufficient sound pressure level.